The present invention relates to a solid-state illumination system, and more particularly, to a solid-state illumination system for use in hospital or other clinical observation areas. It is to be understood, however, that the invention disclosed herein has utility and application in related areas and with additional lighting systems.
Solid-state lighting provides a potentially higher efficiency light source, as compared to conventional discharge-type lamps, and further provides the capability of adjusting spectral characteristics to obtain specific desirable features. Of particular interest herein is the use of solid-state lighting in clinical observation areas, including hospital examination rooms and other clinical settings, where lighting plays an important role in the observation of skin visual appearance to aid in patient assessment.
Clinical observation is an important aspect of assessing a patient's condition, and the available lighting plays a critical role in the accurate assessment of the visual appearance of a patient's skin, including the detection of cyanosis. Cyanosis is a blue coloration of the skin and mucous membranes due to the presence of deoxygenated hemoglobin in blood vessels near the skin surface. Lack of blood oxygenation is an indicator of many potentially harmful medical conditions, some of which may be fatal. Cyanosis can occur in the fingers, as well as other extremities (referred to as peripheral cyanosis), or in the lips and tongue (referred to as central cyanosis).
Fully oxygenated blood generally appears a shade of red. However, when blood is deoxygenated the optical properties of skin distort the dark red color making the skin appear bluish. During cyanosis, tissues that would normally be filled with bright oxygenated blood are instead filled with darker, deoxygenated blood. The scattering of light that produces the blue hue is similar to the process that renders coloration in other objects, i.e. certain wavelengths (colors) dominate the reflected spectrum while others are mostly absorbed. Darker blood absorbs more red wavelengths causing a blue-shifting optical effect, and thus oxygen deficiency leads to an observable blue discoloration of the lips and other mucous membranes.
The color characteristics of lamps used in the electric lighting of hospitals and clinical settings, where observed changes in patient skin appearance are critical, play a significant role in providing the necessary visual conditions for color discrimination-based tasks. In order for cyanosis to be accurately detected, the lighting in such settings should be white, so that the coloration of skin detected by the observing care-giver is not influenced by lighting that inherently casts a dominant hue.
Due to the importance of lighting to accurate patient assessment in clinical observation settings, and more particularly to the need to avoid misdiagnosis of the condition of cyanosis, it was determined that standards should be established to guide hospitals and clinics in choosing appropriate lighting for the purpose of patient observation. The original hospital and medical task lighting standard was established in the early 1970s, based on clinical trials undertaken at the Royal Prince Alfred Hospital in Sydney, to determine optimum lamp color characteristics for tubular fluorescent lamps in use in clinical cyanosis evaluation settings. The result of this initial study was the publication of the standard AS1765: 1975. The lamps at that time were predominantly halophosphate type lamps which exhibit a relatively continuous spectral energy distribution. The results of this trial, established the parameters within which correlated color temperature, and color rendering index values, Ra and R13, should lie to provide light that allows accurate assessment of the presence of cyanosis.
Later, triphosphor lamps were developed. These lamps emit most of their light output in three distinct wavelength bands, with greatly reduced emissions at other wavelengths. The wavelengths of interest for cyanosis detection purposes fall between 620 nm and 700 nm. If the proportion of light emitted in this range is too small, the red coloration of blood is not evident and any change caused by reduced oxygen content may not be seen. Conversely, if there is an excess of light emitted in this range, the patient will always appear well, giving a false result as well. With reference to FIG. 1, which provides a comparison of the spectral power distribution of various lighting sources, it is seen that neither the triphosphor lamp nor the halophosphor lamp generates output in the cyanosis detection wavelength range, i.e., 620 nm-700 nm.
To develop the standard, blood samples having various percentages of oxygenation were tested to determine the spectral reflectance of the blood. The testing was set up to cover 5 nm intervals of emitted light wavelength. In this study, most of the observed changes occurred at wavelengths above 600 nm. This data was then used to determine the difference in color appearance that would result from individual lamps, whether halophosphor or triphosphor, when compared to a reference source comprising a blackbody (Planckian) illuminant having a distribution temperature of 4000° K.
The calculated data was used to render an index to measure the suitability of fluorescent lamps for cyanosis detection. The resulting index, as stated above, is known as the Cyanosis Observation Index (COI). More specifically, the COI is an open ended numerical scale ranking the suitability of a lamp for the purpose of visual detection of the presence or onset of cyanosis. The index is a dimensionless number, calculated from the spectral power distribution of a lamp, and is established by calculating the change in color appearance of fully oxygenated blood, i.e, 100% oxygen saturation, and of oxygen-reduced, cyanosed blood, as assessed by a test lamp, and as compared to a reference lamp. According to the current standard, AS/NZS 1680, lamps exhibiting lower index values are better suited for use in hospital and clinical evaluation settings for detecting the presence of cyanosis. The limiting value on the index is 3.3, with values greater than 3.3 being unacceptable for use in clinical observation settings. Specifically, the standard requires the use of lamps meeting a COI of not more than 3.3, and having a Correlated Color Temperature (CCT) between 3300° K and 5300° K.
It was found that triphosphor lamps are not well suited for this purpose because they have limited emittance in the 600 nm to 700 nm wavelength range where most changes in the reflectance of blood with changing oxygenation take place. This type of lamp generally renders a COI of about 5.3 at 4100° K, well above the limit set by the standard. Cool White halophosphor fluorescent lamps, popular for many other applications and uses, generally exhibit a COI of 15.5.
Correlated color temperature (CCT) is a measure of the “shade” of whiteness of a light source by comparison to a blackbody in equilibrium at a specific temperature. The CCT of typical incandescent lighting is 2700° K which is yellowish-white. Halogen lighting has a CCT of 3000° K. Fluorescent lamps are manufactured to a range of CCT values by altering the mixture of phosphors inside the tube. Warm-white fluorescents have a CCT of 2700° K and are popular for residential lighting. Neutral-white fluorescents have a CCT of 3000° K or 3500° K. Cool-white fluorescents have a CCT of 4100° K and are popular for office lighting. Daylight fluorescents have a CCT of 5000° K to 6500° K, which is bluish-white. CCT can be calculated using the ccx,ccy coordinates of a light source as plotted on the graph shown in FIG. 2, which is the CIE standard chromaticity diagram, as known to those skilled in the art.
The color rendering index (CRI) of a lamp is a measure of its effect on the color appearance of objects in comparison with their appearance under a standard source, such as daylight or a blackbody. Since the spectrum of incandescent lamps is very close to a standard blackbody, they have a CRI of 100. Fluorescent lamps achieve CRI ranging from about 50 to about 95+. Some fluorescent lamps have low red light emission, especially those with high CCT values. These lamps can make skin appear less pink, and hence “unhealthy” as compared to evaluation under incandescent lighting. For example, a 6800° K halophosphate tube (an extreme example) will make reds appear dull red or even brown. Since the human eye is relatively less efficient at detecting red light, light sources with increased energy in the red part of the spectrum, will have reduced overall luminous efficacy.
The COI standard discussed above and set forth in AS/NZS 1680.2: 1997 is used today as a guideline for lighting in hospitals and clinical observation areas where visual observation of a patient's condition is rendered. While some lamps that exhibit acceptable COI values are commercially available, few if any generate a spectrum whose COI is well below the 3.3 standard. One manner of optimizing lamp performance for the purpose of cyanosis detection is to optimize the combination of light sources employed in a lamp or illumination system in order to generate a spectrum of white light whose COI is less than 3.3, preferably less than 2.0, and more preferably less than 1.5. A lamp meeting this lower COI value, if attainable, would provide an observing care-giver with the capability to readily and accurately detect and treat conditions indicated by the presence of cyanosis.
As can be seen from the foregoing, it is critical to patient assessment that lamps selected for use in clinical observation areas meet the COI requirement set in AS/NZS 1680.2. It is further shown that many commercially available lamps prove unsuitable because they exhibit a COI value higher than 3.3, and sometimes much higher.
It would be desirable to have a method to quantifiably predict a combination of light sources that will provide an illumination system capable of generating light that achieves the desired lower COI values, and preferably COI values of less than 2.0 and more preferably less than 1.0, and also meets the required CCT of between about 3300° K and 5300° K. It would also be desirable to have illumination systems that include a combination of light sources meeting this same standard.